Tumors and Cancers in Dogs

A tumor is any sort of lump, bump, growth, or swelling. Tumors that are true cancers are called neoplasms. Tumors can be divided into two broad categories: benign and malignant. Benign tumors grow slowly, don't invade or destroy neighboring tissue, and don't spread to other parts of the body. These cancers aren't usually life-threatening. They are cured by surgical removal, provided that the entire tumor can be removed. Malignant tumors are potentially life-threatening cancers, and are also called carcinomas, sarcomas, or lymphomas, depending on the tissue where they originated.

Cancers invade neighboring tissue and continue to grow in an unrestricted way. At some point, malignant cells part from the primary tumor and enter the lymphatic system or the circulatory system, and establish new colonies in other areas. This process is called metastasizing.

Any new growth on your dog should be examined by a veterinarian. The majority of cancers in dogs are detected by physical examination. About half are visible as growths or sores on or beneath the skin. Perianal tumors, testicular tumors, mammary gland tumors, lymph gland tumors, and cancers in the mouth can be detected by inspection and palpation. Bone tumors can be recognized by a swollen limb, lameness, or the appearance of a swelling that involves the bone.

Internal cancers are most common in the spleen, liver, and gastrointestinal tract. Cancers in these areas often become advanced before they are even suspected. Usually the first signs are weight loss, a palpable mass in the abdomen, vomiting, diarrhea, constipation, or gastrointestinal bleeding. (The signs and symptoms of common tumors affecting the internal organs are discussed in the chapters dealing with these organs.)

Lung cancer is rare in dogs. However, dogs are at risk from secondhand smoke. The lungs are also a frequent site for metastases. The same is true of the liver.

The majority of cancers occur in middle-aged and older dogs. Because companion animals are living longer and enjoying a higher quality of life, it is likely that cancers will be diagnosed with increasing frequency. A routine physical exam will detect most cancers. Regular veterinary visits thus provide the opportunity to detect cancer at an early stage. This has significant implications for the life and health of your pet. As a rule, healthy dogs 7 years of age and older should have a physical examination at least once a year. If their health is questionable, they should be seen more often. If any signs develop (see Danger Signs in the Geriatric Dog, page 546), they should be seen at once.

What Causes Cancer?

Cancer is a condition in which rapid cell division and tissue growth occur at the expense of the host organ. Most cells in the body die and are replaced many times during the course of a dog's life. Cell reduplication follows an orderly pattern controlled by the genes. When things go smoothly, each duplicated cell is an exact clone of its ancestor and assumes the same role.

Anything that disrupts the genes that govern cell duplication results in the production of mutant cells. Mutant cells often reproduce at an extraordinary rate and form large masses that crowd out normal cells. Such a mass is called a cancer. Further, cancerous cells do not function as normal cells and thus do not provide needed services. If the cancer grows unchecked, it eventually replaces much of the organ while also metastasizing to other parts of the body. In time, it causes the death of the dog.

Some cancer-producing genes are inherent in the breed or genetic makeup of a dog. Bernese Mountain Dogs, for example, have a high incidence of cancers affecting all body systems. Approximately one in four Bernese Mountain Dogs will develop cancer; two of the cancer types seen in this breed-histiocytosis and mastocytoma-are known to be inherited as polygenic traits.

A number of genes have been identified as causing breast, colon, and other cancers in people and in some animals. The reason that all individuals with these genes do not develop cancer is that there are other specific genes that suppress the cancer genes. To complicate matters, there are still other genes that inhibit the suppressors. All these genes are turned on and off by external factors, such as diet, stress, and environment. Thus, cancer is a largely unpredictable phenomenon involving a complex interaction of genetics and the environment. A good example is bladder cancer in Scottish Terriers. Scotties have an increased risk of bladder cancer to begin with. If you add in exposure to lawn chemicals that contain 2,4 D, the risk increases four to seven times. In this case, genes and an environmental exposure work together to cause the cancer.

Carcinogens are environmental influences known to increase the likelihood of cancer in direct proportion to the length and intensity of exposure. Carcinogens gain access to tissue cells, cause alterations in genes and chromosomes, and disrupt the system of checks and balances that controls orderly growth. Examples of carcinogens known to increase the risk of cancer in humans are ultraviolet rays (which can cause skin cancers), X-rays (thyroid cancers), nuclear radiation (leukemia), various chemicals (aniline dyes cause bladder cancer), cigarettes and coal tars (lung, bladder, skin, and many other cancers), viruses (sarcoma in AIDS patients), and internal parasites (bladder cancers). Secondhand smoke exposure is associated with cancer in animals as well as in humans.

Injuries are sometimes implicated as causing cancers, but there is seldom a connection. Trauma causes hematomas, bruises, and contusions, but does not cause abnormal cell growth. However, an injured site is usually examined closely, and small preexisting tumors are sometimes discovered this way. Some veterinarians believe bone cancers may be more likely to develop at the site of previous fractures.

Some benign tumors, such as warts and papillomas, are clearly due to a virus. Other benign tumors simply grow for unknown reasons.